The present invention relates to semiconductor devices in which hydrogen is prevented from diffusing into a capacitor insulating film of a ferroelectric film or a high dielectric film forming a capacitor device, and to methods for fabricating such a semiconductor device.
The trend in ferroelectric memory devices is that those of planar structures having a small capacity of 1 to 64 kbit start being produced in volume. Recently, the center of development of the ferroelectric memory devices has been shifting to those of stack structures having a large capacity of 256 kbit to 4 Mbit. To realize the stacked ferroelectric memory device, a significant improvement of the packing density thereof, and by extension microfabrication thereof are indispensable. To attain this, it becomes important to harmonize formation steps of a ferroelectric capacitor, a transistor, and an interconnect.
This harmonization causes a problem in a semiconductor device fabrication process many steps of which are performed in a hydrogen atmosphere as represented by a contact filling technique using W-CVD or heat treatment in the hydrogen atmosphere performed for restoring transistor characteristics. The problem is to retain the polarization properties of a ferroelectric film of a ferroelectric capacitor while no ferroelectric capacitor is reduced.
One of conventionally common techniques is coating of the ferroelectric capacitor with a hydrogen barrier film. In this technique, the hydrogen barrier film represented by an aluminum oxide film or a silicon nitride film shields the capacitor from hydrogen diffusion occurring during the ferroelectric capacitor formation step and later steps in a semiconductor device fabrication process, thereby preventing a decrease in the amount of polarization of the ferroelectric film. The structure in which the hydrogen barrier film covers the entire surrounding of the ferroelectric capacitor is employed as a covering structure of the capacitor, and most effectively prevents degradation of the polarization properties of the ferroelectric film (see, for example, Japanese Patent No. 3098474). Thus, degradation of the polarization properties of the ferroelectric capacitor which is caused by hydrogen is prevented to realize a highly integrated ferroelectric memory device or high dielectric memory device.
Hereinafter, a conventional semiconductor device having a ferroelectric capacitor with the entire surrounding thereof covered will be described with reference to FIG. 11. FIG. 11 is a sectional view of the conventional semiconductor device.
Referring to FIG. 11, in a surface portion of a semiconductor substrate 10, doped layers 11 are formed apart from each other. A gate oxide film 12 and a gate electrode 13 are formed above the semiconductor substrate 10, and sidewalls 14 are formed on side surfaces of the gate oxide film 12 and the gate electrode 13. An isolation oxide film 15 is formed on the semiconductor substrate 10. Above the semiconductor substrate 10, a first interlayer insulating film 16 is formed to cover the gate oxide film 12, the gate electrode 13, the sidewalls 14, and the isolation film 15. A first hydrogen barrier film 17 is formed on the first interlayer insulating film 16.
On the first hydrogen barrier film 17, a ferroelectric capacitor is formed which is composed of a lower electrode 18, a capacitor insulating film 19 made of a ferroelectric film, and an upper electrode 20. A second hydrogen barrier film 21 is formed on the upper electrode 20. On the first hydrogen barrier film 17, a third hydrogen barrier film 22 is formed to cover side surfaces of the ferroelectric capacitor and the second hydrogen barrier film 21. On the first interlayer insulating film 16, a second interlayer insulating film 23 is formed to cover the first hydrogen barrier film 17 and the third hydrogen barrier film 22. Interconnects 24a and 24b are formed on the second interlayer insulating film 23. The interconnect 24a passes through the second interlayer insulating film 23 and the third hydrogen barrier film 22 and is connected to the upper surface of the second hydrogen barrier film 22. The interconnects 24a and 24b pass through the first interlayer insulating film 16 and the second interlayer insulating film 23 and is connected to the upper surface of the doped layer 11.
As shown above, the entire surrounding of the ferroelectric capacitor in FIG. 11 is covered with the first, second and third hydrogen barrier films 17, 21 and 22. Therefore, even though the ferroelectric capacitor is subjected to heat treatment in a reducing atmosphere in the ferroelectric capacitor formation step and later process steps, hydrogen diffusion into the capacitor insulating film 19 can be suppressed. This decreases degradation of the polarization properties of the ferroelectric film forming the capacitor insulating film 19.
When the inventors performed heat treatment in a reducing atmosphere on a ferroelectric capacitor, as mentioned above, with the hydrogen barrier films covering the surrounding thereof, however, it turned out that degradation of the polarization properties of the ferroelectric film forming the capacitor insulating film cannot be prevented completely. In particular, when hydrogen annealing was performed at high concentration, such an incomplete prevention became outstanding.
Hereinafter, this disadvantage will be described concretely with reference to FIGS. 12, 13, 14, 15(a) and 15(b).
The inventors performed heat treatment in a reducing atmosphere on a ferroelectric capacitor, as shown in FIG. 12, with the hydrogen barrier films covering the surrounding thereof.
As shown in FIG. 12, a first interlayer insulating film 31 is formed on a semiconductor substrate 30 in which a memory cell transistor (its illustration is omitted) is formed. A first hydrogen barrier film 32 made of a silicon nitride film is formed on the first interlayer insulating film 31. A second hydrogen barrier film 33 with electrical conductivity is formed on the first hydrogen barrier film 32. On the second hydrogen barrier film 33, a ferroelectric capacitor is formed which is composed of a lower electrode 34 with the surface layer made of a platinum film, a capacitor insulating film 35 made of, for example, an SBT (SrTaBiO) film as a ferroelectric film, and an upper electrode 36 made of a platinum film.
On the first hydrogen barrier film 32, a second interlayer insulating film 37 for smoothing irregularities on the surface of the ferroelectric capacitor is formed to cover side surfaces of the second hydrogen barrier film 33 and the ferroelectric capacitor. On the first interlayer insulating film 31, a third hydrogen barrier film 38 made of a titanium aluminum oxide film is formed to cover side surfaces of the first hydrogen barrier film 32 and the second interlayer insulating film 37. A contact plug 39 is formed to pass through the first hydrogen barrier film 32 and the first interlayer insulating film 31. The contact plug 39 connects the semiconductor substrate 30 to the lower electrode 34 of the ferroelectric capacitor via the second hydrogen barrier film 33.
As shown above, the ferroelectric capacitor in FIG. 12 has the structure in which the entire surrounding thereof is covered with the first, second and third hydrogen barrier films 32, 33 and 38. Therefore, even though the ferroelectric capacitor is subjected to heat treatment in a reducing atmosphere in the ferroelectric capacitor formation step and later process steps, hydrogen diffusion into the capacitor insulating film 35 is suppressed. This prevents degradation of the polarization properties of the ferroelectric film forming the capacitor insulating film 35.
FIG. 13 shows the polarization properties of the capacitor insulating films 35 made of a ferroelectric film when the ferroelectric capacitors shown in FIG. 12 were subjected to heat treatment at 400° C. for ten minutes in atmospheres containing 4% hydrogen and 100% hydrogen, respectively. As is apparent from FIG. 13, in the cases where the ferroelectric capacitors were subjected to heat treatment in the atmospheres containing 4% hydrogen and 100% hydrogen, respectively, the amounts of polarization of the capacitor insulating films 35 formed of a ferroelectric film were decreased. In particular, when the capacitor was subjected to heat treatment in a highly reducing atmosphere as shown in the case of heat treatment in the atmosphere containing 100% hydrogen, it turned out that the extent to which the polarization properties of the ferroelectric film are degraded is large.
FIG. 14 is a TEM cross-sectional view of the contact portion between the first hydrogen barrier film 32 and the second hydrogen barrier film 38 after the heat treatment at 400° C. for ten minutes in the atmosphere containing 100% hydrogen shown in FIG. 13. As is apparent from FIG. 14, the occurrence of a gap was observed in a contact portion 12A between the first hydrogen barrier film 32 made of a silicon nitride film and the third hydrogen barrier film 38 made of a titanium aluminum oxide film.
From the foregoing, the inventors found that degradation of polarization properties of the ferroelectric film results from hydrogen diffusion through the interface at which the hydrogen barrier films come into contact with each other. That is to say, the inventors found that since the extent to which the polarization properties of the ferroelectric film are degraded greatly depends upon the adhesion between the hydrogen barrier films, selection of materials used for the hydrogen barrier films or the state of the contact surface between the hydrogen barrier films coming into contact with each other is of extreme importance.
To carry out a detailed analysis of the state of contact in the above-mentioned contact portion 12A between the first hydrogen barrier film 32 and the third hydrogen barrier film 38, the inventors conducted exemplary experiments using the structure as shown in FIG. 15(a) in which the hydrogen barrier films were in simulated contact with each other.
The structure shown in FIG. 15(a) was formed by sequentially growing above a semiconductor substrate (not shown), a silicon nitride film (a first hydrogen barrier film) and a titanium aluminum oxide film (a second hydrogen barrier film) from bottom to top. The inventors observed the cross section of this structure with a TEM.
From this observation, as shown in FIG. 15(a), the existence of an altered layer with a thickness of about 3.0 nm was recognized in the contact portion (the interface) between the silicon nitride (SiN) film and the titanium aluminum oxide (TiAlO) film.
Furthermore, using EELS (Electron Energy Loss Spectroscopy), the inventors analyzed the altered layer formed at the interface between the silicon nitride film and the titanium aluminum oxide film and the silicon nitride film shown in FIG. 15(a). Then, as shown in FIG. 15(b), a Si—O peak was detected from the result of the analysis of the altered layer. Note that FIG. 15(b) is a TEM cross-sectional view for illustrating the EELS analysis result obtained by the experimental sample of the contact portion between the first and second hydrogen barrier films, and a graph illustrating the relation between the loss energy (eV) and the intensity of the experimental sample.
On the basis of this experimental result, the inventors determined that the altered layer formed at the interface between the silicon nitride film and the titanium aluminum oxide film was a silicon oxide film. This determination results from the assumption that silicon (Si) in the silicon nitride film and oxygen (O) in the titanium aluminum oxide come into contact with each other and they are then subjected to heat treatment in a later process step (e.g., heat treatment for crystallizing a capacitor insulating film), thereby producing stable Si—O bonds.
According to the experimental result described above, it can be considered that even the contact portion between the actual hydrogen barrier films composed of a silicon nitride film and a titanium aluminum oxide film, respectively, is formed with Si—O bonds to create a silicon oxide film.
A silicon oxide film does not have barrier properties of preventing hydrogen entry from the outside. Thus, the contact portion between the silicon nitride film and the titanium aluminum oxide film, which is formed with the silicon oxide film, is sensitive to hydrogen, so that this portion serves to transmit hydrogen from the outside.
Unlike the structure in FIG. 15(a), the contact portion 12A shown in FIG. 14 actually extends in the vertical direction. Therefore, the state of contact between the hydrogen barrier films is poorer than that of the simulatively formed experimental sample. Furthermore, considering that a film stress is easily concentrated on the contact portion 12A, it is expected that Si—O bonds are created in parts of the contact portion 12A. In other words, it is expected that the contact portion 12A is in a condition where silicon oxide films composed of Si—O bonds and gaps are mixed therein.
Hence, in the contact portion 12A, both the region formed with Si—O bonds and the region formed with the gap do not have the hydrogen barrier properties at all. This means that, as shown in FIG. 14, the contact portion 12A serves as a hydrogen diffusion path. Conversely, in the combination of one hydrogen barrier film and the other hydrogen barrier film coming into contact with each other, detection of Si—O bonds in the contact portion between the hydrogen barrier films suggests a high possibility of creating a hydrogen diffusion path.
In the case of the above-mentioned conventional semiconductor device shown in FIG. 11, the first hydrogen barrier film 17 is made of a silicon nitride film with a thickness of 10 to 200 nm which is formed by a low pressure CVD method or a spattering method. The second hydrogen barrier film 21 is made of a titanium nitride film with a thickness of 50 nm which is formed by a spattering method. The third hydrogen barrier film 22 is made of a stacked film formed by sequentially stacking a silicon oxide film and a silicon nitride film from bottom to top, or a film containing oxygen and nitrogen such as a silicon oxynitride film.
Since the stacked film inherently has poor hydrogen barrier properties, it is conceivable that if the stacked film of a silicon oxide film and a silicon nitride film is used as the third hydrogen barrier film 22, however, the polarization properties of the ferroelectric film of the conventional semiconductor device shown in FIG. 11 are degraded to a large extent. Moreover, in the case of the conventional semiconductor device shown in FIG. 11, the structure in which the first, second, and third hydrogen barrier films 17, 21, and 22 cover the entire ferroelectric capacitor is employed for the purpose of reducing degradation of the polarization properties of the ferroelectric film. However, no disclosure is made of which materials are selected for the hydrogen barrier films or how surface treatment is performed on the hydrogen barrier films by noting that the point of view of improving the adhesion between the hydrogen barrier films in contact with each other at the contact portion. Even a discussion from such a viewpoint has not been conducted yet.